This invention relates to an improved method for tightening fasteners which utilizes feedback torque to continuously control the speed and sequence of a fastening tool. This feedback torque is constantly monitored to allow continuous adjustment of the speed of the fastening tool and thereby control the torque applied to the fastener. This method is particularly suited for fastening tools used on fasteners that are required to be fastened within a specific tolerance within a specific time frame. Continuously adjusting the speed of the fastening tool allows a means to optimize the cycle time for fastening.
Typically when tightening fasteners, especially preload setting fasteners, the torque builds rapidly as the fastener approaches its final desired value. Prior fastening tools have been run at a constant speed which often leads to over tightening or exceeding the target torque. This target torque overshoot is a result of the response time of the system controlling the fastening tool along with the speed at which the fastener is tightened. One possible solution has been to use a low speed fastening tool which reduces the overshoot problem, but the time required to tighten a given fastener becomes excessively long.
A more common prior solution has been to use a torque feedback fastening strategy which utilizes a constant speed motor for operating the fastening tool with the addition of a clutch mechanism or reducer gear arrangement which allows modification of the speed of the fastening tool. In most cases these additions allow a two speed or stage operation of the fastening tool. The first stage tightens the fastener at a fast speed to a torque level lower than the desired final torque. The clutch mechanism or reducer gear arrangement is then engaged to start the second stage of tightening. The second stage tightens at a lower speed to allow more accurate control of the fastening tool as the fastener torque approaches the desired final value.
This two stage fastening strategy works well on "soft" joints, i.e., those joints where the joint members have a low spring constant and therefore a high elasticity. Problems arise however when this same two stage strategy is applied to "hard" joints, i.e., those joints where the joint members have a high spring constant and therefore a low elasticity, such as in bearing preload applications. The two major problems encountered in these situations are overshooting the desired torque value and an increased cycle time. The torque overshoot results when the second stage speed is too fast to allow accurate control of the fastening tool. The increased cycle time is usually a result of the torque transducer seeing a premature high torque reading and shifting to the lower fastening speed or shutting off. This can occur when the fastener encounters a burr on the thread as it is tightened which cause a momentary spike in the torque reading which causes the system to shift the fastening tool to its slower speed. The present invention overcomes these problems by providing a novel method for tightening fasteners which utilizes feedback torque to continuously monitor and control the speed of a fastening tool.
U.S. Pat. No. 5,105,519 to Doniwa shows a tension control method for a nutrunner which measures the torque on a fastener at two different angular positions and calculates the ratio of the two values to determine a resultant tension in the fastener which is used to control the starting and stopping of the nutrunner.